1. Field of the Invention
The present invention relates to a microwave oven equipped with a thermopile sensor and a thawing method using the same. In particular, the present invention relates to an improved microwave oven equipped with a thermopile sensor, and a thawing method using the same. The thawing method involves detecting a food surface temperature using the thermopile sensor, optimizing the output from a magnetron of the microwave oven based on the detected food surface temperature, the size of the food and the weight of the same, and determining an optimum thawing completion time, thereby obtaining the best thawing condition and significantly reducing the thawing time.
2. Description of the Conventional Art
FIG. 1 illustrates the construction of a conventional microwave oven.
As shown therein, the conventional microwave oven includes a turn table 30 disposed in a center portion of a heating chamber 20 for placing a frozen food thereon, a magnetron 27 for supplying microwaves over the frozen food through a dope wave guide tube for thawing the frozen food, a turntable motor 29 for rotating the turntable 30, a thermopile sensor 21 disposed at an upper lateral portion of the heating chamber 20 for detecting the temperature of the frozen food and for converting the supplied voltage to a voltage corresponding to the detected temperature, a light 32 for lighting the interior of the heating chamber 20, a cooling fan 28 for cooling the magnetron 27, a microcomputer 22 for receiving a voltage from the thermopile sensor 21, determining thawing time, and outputting a control signal for controlling elements of the microwave oven, and control switches 23 through 26 for turning on/off the light 32, the magnetron 27, the cooling fan 27, and the turntable motor 29 in accordance with a control signal from the microcomputer 22. Additionally, a weight sensor (not shown) is connected to the motor shaft of the turntable for weighing the weight of the frozen food.
The thawing operation of the frozen food using the conventional microwave oven will now be explained with reference to FIGS. 1 through 4C.
The frozen food 31 is placed on the turntable 30 disposed in the heating chamber 20, as shown in FIG. 1, and a front door is closed. When the thawing switch is selected, the microcomputer recognizes the thawing mode, and the thawing operation shown in FIG. 3 is performed as described below.
In step S1, the microcomputer 22 turns on the control switches 23 through 26 for driving the magnetron 27, the cooling fan 28, the turntable motor 29, and the light 32.
The turntable 30 is rotated by the turntable motor 29.
When the turntable 30 is rotated, the microcomputer 22 measures the weight of the frozen food 31 using a weight sensor (not shown) attached to the motor shaft (not shown) of the turntable (step S2).
The time Q elapsing during one rotation of the turntable 30 is computed based on one period time T0 of a supply power and a count P of the turntable motor 29 (step S3), according to: EQU Q=(1/T0)/P.
In the conventional art, the computation is performed assuming P=5 and T0=20 msec, such that Q=10 sec.
After the computation of one rotation time Q of the turntable 29 is finished in step S3, and after a delay of 250 msec in step S4, the microcomputer 22 controls the system so that the magnetron 27 generates a series of outputs corresponding to 0 watts, 300 watts, and 600 watts, as shown in FIG. 2 (step S5).
When the output from the magnetron 27 is controlled and the magnetron 27 is turned off (step S6), the voltage from the thermopile sensor 21 is received. Based on the voltage from the thermocouple, a voltage V which is proportional to the temperature of the frozen food 31 is computed (step S7) as follows: EQU V=R*(V1-V3)+S*V2+T,
where V1 denotes the voltage which is obtained by amplifying the output from the thermopile sensor 21, V2 denotes the voltage of the thermostat, V3 denotes the reference voltage of the thermopile sensor, and R, S and T denote coefficients.
The voltage V corresponding to the temperature of the frozen food 31 is thus computed. Thereafter, and it is checked whether one rotation time (Q seconds) of the turntable 30 has lapsed (step S8). When one rotation time (Q seconds) of the turntable 30 has lapsed, the weight W of the frozen food 31 is measured (step S9).
When the magnetron 27 is turned off and the weight W of the frozen food 31 measured during one rotation of the turntable 30, the amount of time T1 necessary for the magnetron 27 to output 600 Watts is computed (step S10) as follows: EQU T1=0.06*W,
where W represents the weight of the frozen food.
Even though the thermopile sensor 21 does not detect the thawing completion state, the following formulas are used to compute the timing TLmax (hereinafter called a maximum thawing completion time) at which the thawing operation is completed, and the timing TLmin, (hereinafter called a minimum thawing completion time) at which the heating of the magnetron is stopped: EQU TLmax 2*W, EQU TLmin 1*W.
When the maximum and minimum completion timing TLmax and TLmin are obtained, the routine is returned to step S4, and steps S4 through S8 are performed.
In addition, after step S8, when the rotation time (Q seconds) has lapsed after two rotations of the turntable 30, it is checked whether the thawing time is between the minimum thawing completion time TLmin and the maximum thawing completion time TLmax (steps S12 and S13).
If the thawing operation time exceeds the minimum completion time TLmin and the maximum thawing completion time TLmax, the operation is determined to have achieved thawing completion. If the thawing operation time exceeds the minimum thawing completion time TLmin but does not exceed the maximum thawing completion time TLmax, the values L and M are computed as follows (step S14 and S15): EQU L=min/ave, and EQU m=dV/dt,
where min denotes the minimum voltage value which is obtained during one rotation of the turntable, ave denotes the average value, and dV/dt denotes the value which is obtained by differentially computing the voltage V with respect to the time.
The value L is an evaluation value by which the variation amount of the voltage data measured during one rotation of the turntable 30 is computed, and M denotes the value by which it is judged whether the temperature of the food is rapidly increased.
The value L is shown in FIG. 4A. In the case of a large load, the value is shown in FIG. 4B, and when the temperature is within the upper and lower portions of the infrared ray range, namely in the case of a small load, the value L is shown in FIG. 4C.
Therefore, in step S14, the value L is compared with the reference value of 0.094, which is used for judging the variation amount of the voltage data when the minimum thawing completion time TLmin was exceeded but the maximum thawing completion time TLmax was not exceeded.
As a result of the comparison, if the value L is smaller than the reference value of 0.094, the value is presumed to be within the range of an infrared ray as shown in FIG. 4C. Therefore, it is judged that thawing is completed. If the value L is larger than the reference value of 0.094, the value is presumed to correspond to a proper or a larger load, as shown in FIGS. 4A and 4B. Therefore, thawing completion is not assumed. Rather, the size of the value M is compared with the reference value of 10 in step S15 in order to select one of two values.
When the value M is smaller than the reference value of 10, the load is determined to be a load in which the temperature of the center portion of the food is not increased. Therefore, the thawing operation is determined to be completed only after the time reaches the maximum thawing completion time TLmax.
If the value M is larger than the reference value of 10, the load is determined to be a load in which the temperature of the center portion of the food is increased. Therefore, it is judged that the thawing operation is completed.
In the thawing method with respect to the frozen food, the surface temperature of the food 31 is measured by using the thermopile sensor 21. The output from the magnetron 27 is controlled based on the time which is obtained by measuring the weight W of the frozen food by using the weight sensor. Therefore, the thawing completion time is determined.
The food is heated by the high output (600 Watt) during the time of T1 0.06W, which is set in proportion to the weight W of the frozen food measured by the weight sensor. Thereafter, the voltage of 300 Watt is supplied during one rotation (Q seconds) of the turntable 30, and then the voltage of 300 Watt is not supplied during one rotation (Q seconds) of the turntable 30.
Thus, in the conventional art, the weight sensor is used for controlling the output of the magnetron for the time T1 when heating the frozen food by high voltage. Thus, the fabrication and maintenance cost is increased. In addition, when thawing a large amount of the frozen food by using the voltage of 300 Watt after the time T1, a lengthy time is needed to achieve the thawing completion time. Furthermore, since the output of the magnetron is strong compared to a smaller load, the food may be partially heated. In addition, the frozen food may be not evenly heated by an over thawing operation. Still further, if the food to be cooked is eccentrically placed on the turntable, the weight of the food may be erroneously determined, thereby causing malfunction.